Age Rules
The ages of rocks from the lunar highlands vary
widely, making it difficult to test ideas for early lunar
differentiation and crust formation. A team at
Lawrence Livermore National Laboratory and the
University of New Mexico has devised a set of
guidelines to apply to geochronological data that
leads to a relative ranking of the reliability of the age
determined for a sample.
(From Borg et al., 2015, M&PS, v. 50, p. 715-732.)
Applying their guidelines to existing data for lunar
highland rocks shows an upper limit on ages
between 4340 and 4370 Ma. This is essentially the
same as model ages of the formation of KREEP (a
chemical component enriched in potassium, rare
earth elements, and phosphorous) and of the mare
basalt source regions in the mantle.
The numerous ages close to 4370 Ma suggests a
complicated and protracted cooling of the primordial
lunar magma ocean or a widespread vigorous
period of magmatic activity in the Moon.
http://www.psrd.hawaii.edu/Oct15/age-rules.html